Any computer running Windows 7 will be upgradable to Windows 8, Microsoft said today while pledging to keep hardware requirements level or even lower when the next version of Windows comes out.

Microsoft says it has sold more than 400 million Windows 7 licenses, but Windows XP is still nearly twice as commonly used worldwide. Yet Microsoft has already shown two technical previews of Windows 8, and announced today that a further preview of Windows 8 is coming in September. Therefore, Microsoft has a balancing act to convince businesses and consumers to upgrade to Windows 7 despite the promise of a new operating system around the corner.

"Two-thirds of business PCs are still on Windows XP. Moving these users to Windows 7 is important and urgent work for us to get after together," Tami Reller, corporate VP and CFO for Windows, said at Microsoft's Worldwide Partner Conference at the Staples Center in Los Angeles. The conference is Microsoft's opportunity to talk to partners about how they can make money together.

Windows 8 could be released next year, so to prevent businesses from holding on to their cash Microsoft is arguing that users should upgrade now and use the same PC to run Windows 8 later.

"Whether upgrading an existing PC or buying a new one, Windows will adapt to make the most of that hardware," Reller said. Windows 8 is for "the hundreds of millions of modern PCs that exist today and for the devices of tomorrow."

As we learned earlier this year, Windows 8 will be optimized for both touch-screen tablets and PCs. Microsoft announced at January's Consumer Electronics Show that it will support the ARM architecture, a lower-powered chip for mobile devices, and last month Microsoft showed off the new tablet interface.

"Windows 8 is a true re-imagining of Windows, from the chip to the interface," Reller said. Despite the re-imagining, Microsoft will keep system requirements flat or reduce them. To run Windows 7, PCs need at least a 1GHz processor, 1GB RAM, 16GB available disk space and DirectX 9 graphics.

Windows 7 tablets exist today, but regardless of Microsoft's advice, consumers are better served waiting for Windows 8 tablets to hit the market because they are likely to be more advanced and it's not yet clear whether Microsoft can create something better than Apple's iPad. The "buy today, upgrade later" advice should be applied to PCs only.

While a Windows 8 release date hasn't been revealed, Microsoft said today it will provide another technical preview at the BUILD Conference in Anaheim, Calif., Sept. 13-16.

The conference will "show modern hardware and software developers how to take advantage of the future of Windows," Reller said. "It is the first place to dive deep into the future of Windows."

Windows 8 will feature a start screen composed of applications represented in "tiles," which Microsoft believes are more useful than Apple's iPad icons because they are capable of providing details such as the current weather or state of an application. The traditional interface of Windows XP and Windows 7 will also be there for desktop-oriented applications.

Reller claimed Windows 8 will work equally well with touch screens and traditional mouse and keyboard setups, with a browsing experience powered by the next version of Internet Explorer.

"IE10 in Windows 8 will deliver fully optimized touch browsing with all the power of hardware acceleration," she said.

Today, 27% of Web-connected computers run Windows 7, with 51% on Windows XP, according to usage tracker Net Applications.

All versions of the Mac OS X operating system account for just under 5.4%, according to the same group.